The invention relates to a router and to a method of monodirectional amplification of bidirectional optical signals.
In optical transmission systems it is frequently desired to use an optical fibre for bidirectional communication. This is achieved in most practical systems by using wavelength multiplexing so that transmission in one direction takes place at one or more wavelengths, and so that transmission in the other direction takes place at one or more other wavelengths different from the first-mentioned wavelengths.
Since the signals are transmitted through an optical fibre, they will be subjected to attenuation, which necessitates amplification of the optical signals if they are to be transmitted over great distances.
According to the prior art this bidirectional amplification may be achieved by suitable coupling of wavelength multiplex couplers and a unidirectional amplifier. This method, however, is complicated and consequently involves relatively huge costs.
U.S. Pat. No. 5,452,124 discloses a similar system in which a four-part wave ength-division multiplexing filter comprises a multilayer dichroic substrate and self-focusing lenses. The optical signals are connected to two bidirectional ports while the amplifier is connected to two unidirectional ports. Also this solution is complicated and costly.
The router comprises two optical couplers interconnected serially via a delay device and wherein the optical router further comprises an optical amplifier optically connected to one of the optical couplers, a simple and economical router is obtained, which may be designed according to simple dimensioning principles and be adapted to concrete applications. The property that for each optical input an optical coupler ideally divides an arriving optical signal between the outputs of the coupler means that an output signal from the first coupler contains mixed signals, which may subsequently be xe2x80x9cmixed backxe2x80x9d in the following optical coupler. In a suitable embodiment of the delay device, this back-mixing may have the effect that signals with different wavelength components may be fed jointly and selectively to a selected output port on the following coupler, ideally, with conservation of energy, as the interferometer properties of the delay device are utilized.
This complete signal may additionally be fed back into a port on the following coupler, whereby the input ports of the first coupler also serve as output ports.
This property is particularly advantageous in applications where a bidirectional optical signal is to be amplified with a monodirectional amplifier, as a monodirectional amplifier may be coupled between the terminals of the last coupler and amplify both optical signals, following which these, in an amplified state, may be fed back to the bidirectional port of the router. It is noted in particular that the amplified signal is routed to another bidirecional port, for which reason the complete router may be coupled between two fibre ends of a directional light guide cable having a fibre end for bidirectional router ports, amplify arriving optical signals with given wavelengths, and transmit these out on the other bidirectional port to the other fibre end and further on the light guide in the same direction as when it arrived at the router.
When the delay device comprises a difference in distance xcex94L between the two optical guides connecting the two couplers, a simple embodiment of the invention is obtained, as the difference in distance xcex94L provides a mutual phase shift between the two optical signals on the input of the following coupler, which means that the coupler serves as an interferometer in the mixing in the coupler itself.
It will be appreciated that xcex94L is not to be taken to mean a separate physical element, but is an indication of the real MZI difference in distance between the two serially connected couplers.
When 3 dB couplers are used, a particularly simple embodiment of the invention is obtained. The use of 3 dB couplers will usually be preferred, as the characteristic of the complete router is particularly simple when the optical branches of the constituent couplers are symmetrical.
When the delay device is formed by one or more pairs of electrodes arranged along the optical path, a further embodiment of the invention is obtained, wherein a desired phase shift between the optical signals may be achieved by changing the refractive index in the optical path in the delay element in response to an electrical field applied by the electrodes.
When the delay element is provided with one or more pairs of electrodes arranged along the optical path in the delay element to achieve a supplementary time delay, an advantageous embodiment of the invention is obtained, as a desired phase shift between the optical signals may be obtained at an optical,difference in distance xcex94L, and be finely adjusted by changing the refractive index in the optical path in the delay element in response to an electrical field applied by the electrodes.
When xcex94L is equal to xcex2/(2xcex94xcexn) where xcex indicates the optical wavelength used, n is the refractive index, and xcex94xcex indicates the half-period of the power transfer function, i.e. xc2xd FSR (FSR=free spectral range), a practical embodiment of the invention is obtained.
For clarity, it should be mentioned that a selected wavelength of 1550 nm, a refractive index n=1.5, and xcex94xcex=10 nm, result in a difference in distance of xcex94L 80 xcexcm.
When the router is made in an integrated design, an optimum design for commercial use is obtained. This should be taken to mean that the actual design of the delay element is to be made with a relatively great precision, as the necessary distances xcex94L are relatively small, and even small deviations therefrom give rise to a relatively great unreliability with respect to the overall system.
When the optical signals in each direction toward the router are fed to the first bidirectional port A and the second bidirectional port D, respectively, of the router and from there to the first unidirectional port B of the router, further through an optical amplifier connected to the unidirectional ports and from there through the second unidirectional port C of the router and back through the router to the second bidirectional D and the first bidirectional port A, respectively, an effective bidirectional amplification is obtained, using relatively inexpensive elements. The bidirectional amplification obtained is moreover obtained using just one monodirectional amplifier.
When xcexr1 and xcexr2 are allocated on the power transfer function of the router in one transmission direction on each side of a maximum of xcexR, and xcex11 and xcex12 are allocated on the power transfer function of the router in the other transmission direction on each side of a maximum of xcexL, said bidirectional optical signals having the wavelengths xcex11 and xcex12 in one direction and having the wavelengths xcexr1 and xcexr2 in the other direction, said xcexL and xcexR indicating a maximum in a specific frequency band for the power transfer function of the router in one direction and the power transfer function of the router in the other direction, respectively, an effective amplification of a bidirectional signal is obtained, using a relatively simple and inexpensive technique, as a two-channel signal may thus be transmitted and amplified each way through the router.